Until the industrial revolution of the turn from the eighteenth century to the nineteenth century, books were almost universally printed on rag paper. Rag paper lasts for centuries, but was and still is expensive. The nineteenth century has provided mankind with inexpensive paper made of chemically treated wood chips ("wood pulp"), thereby placing books and newspapers within the economic reach of nearly everyone. Paper of wood origin cannot be printed or written upon, unless it is first treated with a sizing composition. A typical sizing composition is a mixture of rosin and alum (aluminum sulfate, Al.sub.2 (SO.sub.4).sub.3). Alum readily reacts with moisture and generates sulfuric acid (H.sub.2 SO.sub.4), a strong acid which contributes significantly to acid-catalyzed hydrolysis of the cellulose content of the paper and thereby embrittles and otherwise weakens and degrades it within about 5 to 75 years in the case of books, and in the case of newsprint even within a few weeks or months. Industrial pollution has aggravated the sulfuric acid problem for printed paper.
Other acids, such as the weak acid acetic acid (CH.sub.3.COOH, sometimes written as HOAc) and its analogs are also found or formed in paper made from wood pulp and attack it.
The acidity of printing paper of wood origin is usually in the range of pH 4 to 5. A mildly acidic pH of about 6 is fairly harmless, the danger point for printing wood pulp paper being about 5.5. A neutral pH of 7 for such printing paper would be optimal but is not easy to maintain unless alkaline materials are added. Mild alkalinity in printing paper, such as a pH of about 7.5 to 8.5 from the additive, has the advantage of providing an "alkaline reserve" to furnish a margin of safety and protect the pH of the paper from going too far on the acid side and below the pH 5.5 danger point. However, excessive alkalinity in printing paper, i.e. above pH 11, should also be avoided.
Numerous chemical treatments to reduce the acidity of commercial printing paper are known. The Preservation Office of the Library of Congress, Washington, D.C. has published in 1991 a "Bibliography on Mass Deacidification," listing over 250 texts (books, articles, representative patents, etc.) up to 1990. An ideal paper deacidification treatment, combining permanence, cost effectiveness and safety has not yet been found.
The Library of Congress, as the world's greatest repository of books and manuscripts, is currently developing a mass paper deacidification treatment with the vapor of diethyl zinc (DEZ), which has the advantage of effectiveness and permanence. This treatment is covered by U.S. Pat. Nos. 3,969,549 (1976) and 4,051,276 (1977), Williams and Kelly, both assigned to the Government of the United States as represented by the Librarian of Congress.
Diethyl zinc in vapor form is a safe chemical when handled properly. However, this process requires skillful handling because diethyl zinc is usually shipped in the liquid state and in that state tends to react violently with water and has pyrophoric properties. Also diethyl zinc is presently somewhat costly. A 1988 publication by the Office of Technology Assessment (OTA) of the Congress of the United States entitled "Book Preservation Technologies" contains (pp. 54/55 and 111) an estimate by the Library of Congress of operating costs of a DEZ deacidification facility for a throughput of one million books per year, at about $1.82 per book (not including plant capital recovery and the handling and transportation of books outside the deacidification facility).